Monday, June 3, 2013

On 30 May 2013 the press reverberated with the news
that the Near-Earth Asteroid 1998 QE2, a monster “the size of nine ocean liners”,
was going to sail by Earth (without docking).This is a terrifying image: picture nine ocean liners falling from the
sky!What a splash that would make! And some sources say it is long as nine ocean liners,
and some say the width of nine ocean liners, and some say as big as nine ocean
liners…well, whatever.

Of course the comparison of an asteroid with an
ocean liner was motivated by the asteroid’s moniker (QE2), and should be
understood as poetic license, or at least as unlicensed poeticism.But is an asteroid of this size really a big
threat?True, it missed us by 5.8 million
kilometers THIS time, but its orbit will bring it back across Earth’s orbit over
and over again.We can’t guarantee we
will always be so lucky!

The realists among us will appreciate that the news
media routinely push the envelope of truth in order to generate scary
headlines.This is usually done by means
of liberal use of adjectives (huge, gigantic, etc.; strangely I haven’t seen
anyone refer to it as Titanic) rather than numerical facts.So, assuming we know about the existence and
usefulness of numbers, does QE2 represent a real threat? Good question… and thanks for asking!

1998 QE2 is in fact about 1.7 miles in diameter, a
pretty decent piece of rock, and has a 2000-foot satellite in orbit around it.Now, let’s see: 1.7 miles is about 2700
meters in diameter, or a radius of 1350 meters and a volume of 10.3
billion cubic meters.At an average
meteorite or asteroid density of 3 tonnes per cubic meter, this is 31 billion
tonnes.Now let’s compare it to a big
ocean liner—for example the Queen Elizabeth II:displacement 44,000 tonnes.So,
by the difficult mathematical operation known as long division, we see that
this asteroid (never mind its satellite) would deliver a mass equivalent to 700,000
ocean liners onto our unsuspecting heads. WHAT?The
sensationalist media have underplayed the
story by a factor of 80,000?

Why the disconnect?Because of the vague use of the word “size”.Some people use it to mean length, or area,
or volume, or mass.The length of an
asteroid tells little about the size of a threat it presents—unless, of course,
we know how to do arithmetic.

But the real measure of its potential for wreaking
havoc is its total kinetic energy.Let’s
take an impact speed of 16 kilometers per second as an example (many NEAs are
moving a lot faster than that).That
puts it in the million-megaton (1
teraton) league.That would be
comparable to 100 World War IIIs.

John S. Lewis

John S. Lewis is a professor emeritus of planetary science at the University of Arizona’s Lunar and Planetary Laboratory and is Chief Scientist at Deep Space Industries. His interests in the chemistry and formation of the solar system and the economic development of space have made him a leading proponent of turning potentially hazardous near-Earth objects into lucrative space resources. Prior to joining the University of Arizona, Lewis taught space sciences and cosmochemistry at the Massachusetts Institute of Technology. He received his education at Princeton University, Dartmouth College and the University of California, San Diego, where he studied under Nobel Laureate Harold Urey. An expert on the composition and chemistry of asteroids and comets, Lewis has written such popular science books as "Mining the Sky", "Rain of Iron and Ice", and "Worlds without End".